This invention relates generally to a programmable cardiac stimulation apparatus for the purpose of monitoring the progression of congestive heart failure or the efficacy of delivered heart failure therapies. More specifically, the present invention is directed to an implantable stimulation device and associated method for monitoring and analyzing physiological parameters indicative of overall patient well-being in order to provide diagnostic information for heart failure therapy optimization.
Congestive heart failure (CHF) is a debilitating, end-stage disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the body""s tissues. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract. Often, the ventricles do not adequately fill with blood between heartbeats and the valves regulating blood flow may become leaky, allowing regurgitation or back flow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness, and inability to carry out daily tasks may result. Not all CHF patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive.
As CHF progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds muscle causing the ventricles to grow in volume in an attempt to pump more blood with each heartbeat. This places a still higher demand on the heart""s oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output.
CHF has been classified by the New York Heart Association (NYHA). Their classification of CHF corresponds to four stages of progressively worsening symptoms and exercise capacity from Class I to Class IV. Class I corresponds to no limitation where ordinary physical activity does not cause undue fatigue, shortness of breath, or palpitation. Class II corresponds to slight limitation of physical activity where such patients are comfortable at rest, but where ordinary physical activity results in fatigue, shortness of breath, palpitations, or angina. Class III corresponds to a marked limitation of physical activity where, although patients are comfortable at rest, even less than ordinary activity will lead to symptoms. Class IV corresponds to inability to carry on any physical activity without discomfort, where symptoms of CHF are present even at rest and where increased discomfort is experienced with any physical activity.
Current standard treatment for heart failure is typically centered around medical treatment using ACE inhibitors, diuretics, and digitalis. It has also been demonstrated that aerobic exercise may improve exercise tolerance, improve quality of life, and decrease symptoms. Heart transplantation is an option, but only in 1 out of 200 cases. Other cardiac surgery may also be indicated, but only for a small percentage of patients with particular etiologies. Although advances in pharmacological therapy have significantly improved the survival rate and quality of life of patients, patients in NYHA Classes III or IV, who are still refractory to drug therapy, have a poor prognosis and limited exercise tolerance. Cardiac pacing has been proposed as a new primary treatment for patients with drug-refractory CHF.
By tracking the progression or regression of CHF more closely, treatments could be administered more effectively. Commonly, patients adapt their lifestyle and activities to their physical condition. The activity level of the patients with NYHA Class III or IV would be much lower than that of the patients with NYHA Class I or II. The change in lifestyle or activity level, due to the patient""s heart condition, will be reflected by activity and respiration physiological parameters.
Besides various assessments of the cardiac function itself, assessment of activity and respiration are typically performed. This includes maximal exercise testing in which the heart rate and maximum ventilation are measured during peak exertion. However, peak exercise performance has been found to not always correlate well with improvements in a patient""s clinical condition. Therefore, sub-maximal exercise testing can also be performed, such as a six-minute walk test. While improvements in sub-maximal exercise may suggest an improvement in clinical condition, sub-maximal exercise performance can be variable in that it is dependent on how the patient happens to be feeling on the particular day of the test.
To obtain a more general assessment of the patient""s activity on a daily basis, patients are often asked to answer questionnaires regarding numerous aspects of daily life. Such questionnaires are inherently subjective. Nevertheless, collected information is useful to the physician. Since existing CHF treatments are palliative and not curative, a major goal in administering therapies is to improve the quality of daily life which is directly reflected by the level and variety of activities the patient is comfortable performing.
Thus, it would be desirable to have an objective means of chronically and non-invasively monitoring physiological parameters indicative of a patient""s overall well-being on an ongoing, daily basis. This would enhance the physician""s ability to optimize and carefully tailor therapies for stabilizing CHF.
A number of attempts have been made previously to provide for chronic monitoring of physiological parameters associated with CHF using implantable cardiac devices, such as pacemakers, in conjunction with physiological sensors. Reference is made to U.S. Pat. No. 5,518,001 to Snell; U.S. Pat. No. 5,944,745 to Rueter; U.S. Pat. No. 5,974,340 to Kadhiresan; U.S. Pat. No. 5,935,081 to Kadhiresan; U.S. Pat. No. 6,021,351 to Kadhiresan et al.; and U.S. Pat. No. 5,792,197 to Nappholz. Reference is also made to U.S. Pat. No. 4,901,725 to Nappholz, et al.; and U.S. Pat. No. 5,964,788 to Greenhut, that generally describe rate-responsive pacemakers using impedance measurements of respiration for controlling the pacing rate.
However, there is still an unsatisfied need for a method of chronically and objectively monitoring related physiological indicators of the severity of CHF, at time points representative of the overall patient condition, to thereby reflect a worsening or improving condition associated with therapy delivery. This method would also permit reporting and displaying resulting data in a way that is useful and informative to the physician.
One feature of the present invention to satisfy this need is to monitor physiological parameters associated with the progression, stabilization, or regression of symptoms of heart disease such as congestive heart failure (CHF). The monitoring is implemented by ongoing surrogate measurement of standard and direct measurements, such as daily activity and respiratory and cardiac rate response, utilizing existing implantable, rate-responsive stimulation devices that incorporate activity, respiration, and/or other sensors.
To further optimize CHF therapy, the present invention provides a method of processing the collected data and displaying relationships of the measured parameters in a way that is diagnostically meaningful to the physician. These goals are achieved without significant memory requirements, complex circuitry, or additionally implanted hardware.
In one embodiment of the present invention, a piezoelectric accelerometer measures activity (ACT) when triggered by changes in the sensed intrinsic heart rate (HR), or changes in a sensor-indicated pacing rate (SIR). In another embodiment, the present invention uses an impedance measurement to monitor respiration, more specifically to monitor increases in minute ventilation (MV) above an average resting minute ventilation, when triggered by changes in the sensed heart rate or changes in the sensor-indicated pacing rate. The activity and the minute ventilation data can be collected simultaneously, such that the level of daily activities can be correlated to both respiratory and cardiac rate responses.
Another aspect of the present invention is to provide a method for processing and displaying the measured activity or minute ventilation data to interpolate diagnostic relationships between activity, minute ventilation, heart rate, or sensor-indicated pacing rate, that are representative of the overall well-being of the patient, thus reflective of the severity of CHF symptoms. Activity and minute ventilation data collected upon each heart rate or sensor-indicated pacing rate change are stored in histogram bins assigned to defined heart rate or sensor-indicated pacing rate ranges. After a given period of data collection, such as 24 hours, the data for each rate range is averaged and statistical or mathematical analysis is performed to determine correlation or regression coefficients that define the relationships between activity, heart rate, sensor-indicated pacing rate, or minute ventilation. Storing the averages and the relationship coefficients allows for future graphical display of the periodic data and frees memory bins for the next data collection time interval.
Thus, a further aspect of the present invention is a method that allows data to be downloaded and displayed in a diagnostically meaningful way during a routine office visit without requiring significant technical expertise and without additional invasive, time-consuming or expensive procedures. Such diagnostic data are valuable to a physician in adjusting medical or pacing therapies for the treatment of CHF.
Another aspect of the present invention allows for metabolic monitoring when neither the intrinsic heart rate nor the sensor-indicated pacing rate is available, e.g., during fixed rate pacing modes. During such modes, changes in the level of one sensed parameter, for example activity, can be used to trigger collection and data storage of one or more other sensed parameters, for example minute ventilation. In this way, a change in activity level triggers collection of minute ventilation data that are stored in memory according to defined ranges of activity. Minute ventilation and activity data can thus be collected, and an inter-relationship determined and graphically displayed during periods of fixed rate pacing.